U.S. patent number 4,569,695 [Application Number 06/602,519] was granted by the patent office on 1986-02-11 for method of cleaning a photo-mask.
This patent grant is currently assigned to NEC Corporation. Invention is credited to Toshio Wada, Hiromi Yamashita.
United States Patent |
4,569,695 |
Yamashita , et al. |
February 11, 1986 |
Method of cleaning a photo-mask
Abstract
A photo-mask to be used in a light exposure step for
manufacturing semiconductor devices is cleaned by wetting front and
rear surfaces of the mask with a liquid, brushing the wetted
surfaces with a pair of rotary brushes, wetting the brushed
surfaces with an electrolytic solution containing sufficient
electrolyte to substantially eliminate electrostatic charge from
the surfaces, spraying and immersing the photo-mask in an organic
liquid such as an alcohol, and then drying the photo-mask.
Inventors: |
Yamashita; Hiromi (Tokyo,
JP), Wada; Toshio (Tokyo, JP) |
Assignee: |
NEC Corporation (Tokyo,
JP)
|
Family
ID: |
26411648 |
Appl.
No.: |
06/602,519 |
Filed: |
April 20, 1984 |
Foreign Application Priority Data
|
|
|
|
|
Apr 21, 1983 [JP] |
|
|
58-70494 |
Apr 21, 1983 [JP] |
|
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58-70495 |
|
Current U.S.
Class: |
134/1; 134/2;
134/28; 134/29; 134/3; 134/30; 134/31; 134/6; 134/9; 15/1.51;
15/77 |
Current CPC
Class: |
B08B
11/00 (20130101); G03F 1/82 (20130101); H01L
21/67057 (20130101); H01L 21/67046 (20130101); H01L
21/67051 (20130101); H01L 21/6704 (20130101) |
Current International
Class: |
B08B
11/00 (20060101); H01L 21/00 (20060101); B08B
001/02 (); B08B 001/04 (); B08B 006/00 (); B08B
007/04 () |
Field of
Search: |
;134/2,3,9,28,29,30,31,199,1,6 ;15/77,1.5R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Caroff; Marc L.
Attorney, Agent or Firm: Burns, Doane, Swecker &
Mathis
Claims
What is claimed is:
1. A method of cleaning a photo-mask to be used in a light exposure
step for manufacturing semiconductor devices comprising the steps
of wetting front and rear surfaces of said mask with liquid,
passing said mask through a gap between a pair of rotary brushes
and brushing said front and rear surfaces with said rotary brushes,
making an electrolytic solution flow along said front and rear
surfaces of said mask, said electrolytic solution containing
sufficient electrolyte to substantially eliminate electrostatic
charge from said surfaces, making an organic liquid flow along said
front and rear surfaces of said mask, and drying said front and
rear surfaces of said mask.
2. A method of claim 1, in which said liquid in said first step is
a dilute solution of ammonia.
3. A method of claim 1, in which said liquid in said first step is
a weak acidic aqueous solution containing carbonic acid.
4. A method of claim 1, in which said electrolytic solution in said
third step is a dilute solution of ammonia.
5. A method of claim 1, in which said electrolytic solution in said
third step is a weak acidic aqueous solution containing carbonic
acid.
6. A method of claim 1, in which said organic liquid in said fourth
step is an alcohol.
7. A method of claim 1, in which said drying step is conducted by
contacting the photo-mask with an organic gas.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of cleaning a light
exposure mask, that is a photo-mask, to be used for manufacturing
of semiconductor devices such as IC's, LSI's, VLSI's, etc.
(hereinafter called simply "mask") and a cleaning apparatus for
practicing the method.
2. Description of the Prior Art
One of the important steps in a manufacturing process of
semiconductor devices is a light exposure step. The mask to be used
in this step must be free from dust or dirt. More particularly,
straying dust would adhere on the front and rear surfaces of the
mask because the electric charges are accumulated in the patterns
on the mask by irradiating ultraviolet rays or far-ultraviolet rays
in this step. In one method of cleaning the mask in the prior art,
only brush-cleaning of either one or both of the front and rear
surfaces of the mask was performed with the aid of a neutral
detergent or pure water, and thereafter drying was effected by
means of Freon. Alternatively, in another method of cleaning of the
mask in the prior art, only shower-cleaning of either one or both
of the front and rear surfaces of the mask was performed with the
aid of a neutral detergent or pure water, and thereafter drying was
effected by means of Freon. However, the former method would cause
electrostatic breakdown in the pattern on the mask and also had a
poor cleaning effect. The latter method was poorer in the cleaning
effect than the former method. Consequently, there was a posibility
that the influence of dust on the mask cleaned by the prior art
method would be generated in semiconductor clips of 5% or more in a
semiconductor wafer. Furthermore, in a reduced-size projection
light exposure apparatus, a reticle mask used in the apparatus must
be free from dust of 3 .mu.m or more in size and stains or
contaminations produced upon drying of a cleaning liquid. However,
a mask cleaning technique which fulfils such type of requirements
and which has a good workability has not been known. On the other
hand, to inspect a cleaning effect, that is, an extent of removal
of dust and the like in a mask after cleaning, is an important
factor for dertermining the product yield of semiconductor
elements. Quality control of a mask after cleaning in the prior
art, however, was directly effected in some cases by means of a
microscope, and in other cases it was effected by mounting the mask
to a light exposure apparatus, carrying out light exposure of a
semiconductor substrate coated with a photo-resist and then
observing the produced photo-resist pattern by means of a
microscope. The time and labor necessitated for this observation
are immeasurable.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a method of
cleaning a mask which method is excellent in workability and can
reliably remove dust and the like adsorbed on the front and rear
surfaces of the mask, and an apparatus for practicing the same
method.
According to one feature of the present invention, there is
provided a method of cleaning a photo-mask, such as a reticle mask,
to be used in a light exposure step for manufacturing semiconductor
devices and having conductive or semiconductive patterns on a
transparent substrate made of glass or quartz, the method
comprising steps of a first step of wetting the front and rear
surfaces of the mask with liquid by a shower-like manner, a second
step of passing the mask through a gap between a pair of rotary
brushes, a third step of making an electrolytic solution flow along
the front and rear surfaces of said mask by a shower-like manner, a
fourth step of making an organic liquid such as alcohol flow along
the front and rear surfaces of said mask, and a fifth step of
drying the front and rear surfaces of said mask after bringing the
front and rear surfaces into contact with a vapor of an organic gas
such as Freon. The liquid in the first step is favorably a weak
acidic aqueous solution such as carbonic acid or a weak alkaline
aqueous solution such as a dilute solution of ammonia. However, a
weak acidic aqueous solution of phosphoric acid, hydrochloric acid,
etc., or alcohol may be used. In the second step, adsorbed matters,
adhering matters, contaminations, stains, etc. on the front and
rear surfaces of the mask can be mechanically removed. The mask has
been wetted with electrolytic solution such as dilute solution of
ammonia in the first step, charges would not be accumulated in the
second step of mechanical scrubbing step. Moreover, the third step
is conducted for eliminating electrical charges completely. The
electrolytic solution in the third step employs the same solution
in the first step. In addition, in the case of employing an
alkaline liquid such as ammonia, sodium hydroxide, potassium
hydroxide, etc. as the electrolytic solution, fingerprints, oily
contaminations and the like adhering onto the mask can be removed.
Furthermore, owing to the fact that an aqueous solution remaining
upon cleaning a mask is replaced by alcohol in the fourth step and
vapor cleaning-drying by means of an organic gas in the fifth step
is effected, a drying speed is fast and residual stains as will be
observed upon natural drying of an aqueous solution can be
perfectly eliminated.
According to another feature of the present invention, there is
provided a method of cleaning automatically a photo-mask to be used
in a light exposure step for manufacturing semiconductor devices
comprising steps of containing the mask in a protecting case,
inserting the protecting case into a case accommodating section,
taking out automatically the mask from the protecting case and
supporting automatically the mask by support member at its
pheripheral portion so as to expose the most part of the front and
rear surfaces of the mask in the accommodating section,
transporting automatically the mask with the support member to a
cleaning section, cleaning automatically the mask in the cleaning
section, transporting automatically the mask with the support
member to a dust inspection section, inspecting automatically the
mask in the dust inspection section, transporting automatically the
mask with the support member to the case accommodating section when
the result of the inspection is acceptable and to the cleaning
section to re-clean the mask when the result of the inspection is
unacceptable, releasing automatically the acceptable mask from the
support member and setting automatically it into the protecting
case, and taking out the protecting case containing the acceptable
mask from the case accommodating section.
According to another aspect of the present invention, there is
provided an apparatus for cleaning a photo-mask to be used for
manufacturing of semiconductor devices, comprising means for
holding at least a part of the periphery of the mask, mechanical
cleaning means for wetting the surfaces of the mask with liquid and
brushing the surfaces, means for spraying an electrolytic solution
onto the front and rear surfaces of the mask, means for spraying
organic chemicals onto the front and rear surfaces of the mask, and
means for vapor-cleaning and drying the mask by means of an organic
gas.
According to further aspect of the present invention, there is
provided an apparatus for cleaning a photo-mask, comprising means
for bringing the mask to be cleaned into a cleaning section and
taking it out of the cleaning section after cleaning, means for
detecting dust remaining on the taken-out mask and determing
whether the result of cleaning is good or bad, means for again
bringing the mask into the cleaning section in the event that the
result is bad, and means for bringing the mask into mask protecting
case within a mask accommodaing section in the event that the
result is good.
According to the present invention, the cleaning apparatus is
provided with an automatic dust detection and determination
capability, which makes it possible to detect dust remaining on a
mask after cleaning, for example, by scanning the mask with laser
rays and sensing a light scattered by the dust, and to repeatedly
clean the mask on the basis of the information obtained by the
detection of the dust until an acceptable mask is obtained, and
further a capability of automatically accommodating the acceptable
mask obtained as a result of the cleaning or the repeated cleaning
in a mask protecting case for protecting the acceptable mask from
straying dust or the like. Moreover, since inspection of a mask
after cleaning is performed automatically by means of laser rays,
the inspection time is shortened and the inspection can be carried
out accurately. Still further, in the event that the result of
inspection proves that a predetermined amount of dust still adheres
to the mask, the mask is again sent to a cleaning section
automatically to be cleaned, and hence, a mask coming out of the
cleaning apparatus according to the present invention is a mask for
which predetermined cleaning has been completed. Accordingly, a
predetermined yield can be obtained for the semiconductor devices
fabricated by making use of a mask cleaned according the present
invention. In addition, upon application to a reduced-size
projection type light exposure apparatus (stepper), since an
acceptable mask can be directly handled while it is kept
accommodated in a mask protecting case, it has become possible to
eliminate the possibility of adhesion of every dust during the
period from confirmation of an acceptable mask after cleaning until
mounting of the mask to a stepper, common defects in a repeated
pattern to be transferred onto a semiconductor substrate can be
easily removed, and hence it becomes possible to greatly improve a
utilization efficiency of a stepper. Accordingly, practical
advantages of the present invention are very great.
BRIEF DESCRIPTION OF THE DRAWINGS
One preferred embodiment of the present invention is illustrated in
FIGS. 1 to 6 of the accompanying drawings, wherein:
FIG. 1 is a block diagram showing relations between respective
sections,
FIG. 2 is a schematic view showing an outline of relative
arrangement of the respective sections,
FIG. 3 is a perspective view showing the mode of loading and
unloading a mask into and from a protecting case,
FIG. 4 is a front view showing the mode of supporting a mask by
means of a support member in a cleaning section, in an inspecting
section and in respective transport sections,
FIG. 5 is a schematic view showing a cleaning section, and
FIG. 6 is a schematic view showing an inspection section.
DESCRIPTION OF A PREFFERED EMBODIMENT
At first, an outline of the present invention will be described
with reference to FIGS. 1 and 2. As indicated by arrow 11, a
protecting case containing a mask therein is inserted in a case
accommodating section 4. Next, the mask is passed through an
automatic dust inspecting section 3 and is sent to a mask cleaning
section 2. This flow is indicated by arrow 12. It is to be noted
that during the above-mentioned step, either dust on a mask before
cleaning could be checked or could not be checked in the automatic
dust inspecting section 3. A mask having completed cleaning in the
cleaning section 2 is sent to the automatic dust inspection section
3 as shown by arrow 13, and it is inspected in this section.
Through this inspection, an amount and sizes of dust remaining on
the mask are quantitatively detected, and a mask fulfilling preset
reference conditions is determined to be acceptable, hence is sent
to a case accommodating section 4 as shown by arrow 12' and is
again loaded into a protecting case and discharged externally as
shown by arrow 11'. On the other hand, in the event that a mask has
been determined to be unacceptable, again it is sent to the
cleaning section 2 as shown by arrow 13' to be subjected to
cleaning, then inspection of residual dust is again performed, and
if the mask is again determined to be unacceptable, similar
cleaning and dust inspection are repeated. If a mask can be
determined to be acceptable within a preset limit number of
repetitions of the cleaning and dust inspection, then it is
automatically loaded into an mask protecting case, and thereby the
cleaning by means of the apparatus according to the present
invention is completed. In other words, transport between the
respective sections as well as processings in the respective
sections are all performed automatically. The results of the dust
inspection are output as indicated by arrow 14, for instance, is
displayed on a CRT 5 to be confirmed by an operator, and further
they are stored as inspection data.
FIG. 3 shows the mode of loading and unloading a mask 6 into and
from a protecting case 9 within a case accommodating section. In
more particular, a side lid portion 9' of the protecting case 9 is
opened and closed by means of a motor 15 and a shaft 14 connected
thereto. When this lid portion 9' is opened, a projected fork 10'
of a fork lift mechanism is inserted into the protecting case 9 to
take out a mask 6 as placed on the fork 10'. When a mask 6 is to be
loaded into the protecting case 9 also, a similar operation is
performed. The mask 6 is automatically transferred from this fork
lift mechanism 10 to a support member 7 shown in FIG. 4. With
reference to FIG. 4, the support member 7 is provided with rotary
claws 8, and these claws 8 also can be rotated automatically. This
support member 7 supports only a peripheral portion of a mask 6,
and it carries out the transfers at the locations indicated by
arrows 12, 12', 13 and 13' and the processings in the inspecting
section 3 and the cleaning section 2 in FIGS. 1 and 2, while
maintaining the most part of the front and rear surfaces of the
mask 6 in an exposed condition. The support member 7 is generally
transported by a robot, and it is rotatable so that in the
inspecting section 3 the mask 6 may be held horizontally, while in
the cleaning section 2 it may be held vertically.
FIG. 5 schematically shows the cleaning section 2. In this figure,
masks are designated by reference numerals 101, 102, 103 and 104.
The support member shown in FIG. 4 and a robot for carrying out an
automatic processing are omitted from FIG. 5 for the purpose of
avoiding complexity. In the illustrated embodiment of the cleaning
section, there are provided four cleaning tanks 20, 30, 40 and 50
for cleaning, in the successive steps, masks 101, 102, 103 and 104
to be used for fabrication of semiconductor devices, which masks
are formed by depositing a shading pattern of a necessary
configuration made of chromium on one surface of a transparent
quartz substrate. The respective cleaning tanks can simultaneously
clean the front and rear surfaces of a mask to be cleaned, and to
the end, the mask is held at its at least one end portion by means
of an appropriate jig such as the support member 7 shown in FIG. 4
and is moved up and down within the respective cleaning tanks.
In a first cleaning tank 20, a mask is passed through a gap between
a pair of opposed shower pipes 21 and 21' for the purpose of
wetting the front and rear surfaces of the mask 104 to be cleaned,
with aqueous ammonia of 1% or less. The mask passed through this
gap is further passed through a gap between a pair of rotary bushes
22 and 22' made of nylon to have its front and rear surfaces
scrubbed thereby. The directions of rotation of these rotary
brushes are such directions that they may sweep over the mask
surfaces from the top to the bottom as shown in FIG. 5. In
addition, the rotary brushes 22 and 22' may spray, in some cases,
similar liquid to that sprayed from the pipes 21 and 21', from
their center portions towards the mask surfaces. In this way, in
this cleaning tank 20 are carried out the first step of wetting the
front and rear surfaces of the mask and the second step of
mechanically sweeping over the front and rear surfaces of the mask
by means of the rotary brushes.
Subsequently, the mask to be cleaned is subjected to shower
cleaning with an electrolytic solution in a second cleaning tank
30. In this cleaning tank 30, necessarily an electrolytic solution
flows out of a pair of shower pipes 31 and 31', and by passing the
mask 103 through the gap between these shower pipes, the
electrolytic solution is made to flow along the front and rear
surfaces of the mask. In this third step performed in the second
cleaning tank 30, electrostatic charge accumulated during the brush
cleaning in the preceding stage as well as during use of the mask
is completely removed, and thereby dust on the front and rear
surfaces of the mask is released from static electricity and
removed by the flowing electrolytic solution. For the electrolytic
solution to be used in this step, aqueous ammonia or aqueous
carbonic acid having a concentration of 0.1% to about 2% is
conveniently available. The electrolytic solution flowing through
the shower pipes 31 and 31' in the second cleaning tank 30 may be,
if necessary, switched to pure water after finishment of the
cleaning to rinse out the electrolytic solution adhering onto the
front and rear surfaces of the mask. The above-described
concentrations of the electrolytic solutions are represented in
weight %.
In a third cleaning tank 40, for the purpose of replacing the
aqueous solutions used in the preceding steps by an organic liquid,
a mask 102 to be cleaned is passed through a gap between a pair of
shower pipes 41 and 41', then it is immersed in an isopropyl
alcohol bath 42, and then, after it is again sprayed with isopropyl
alcohol ejected from the shower pipes 41 and 41', it may be
delivered to a fourth cleaning tank 50. The alcohol used in this
step is not limited to isoprophy alcohol, but other alcohol series
chemicals such as methanol, ethanol, etc. could be employed. The
alcohol bath 42 is supplied with the alcohol ejected from the
shower pipes 41 and 41', and the alcohol would overflow beyond all
the wall edges on the four sides and then would flow out
externally.
A mask 101 to be cleaned which has been delivered to the fourth
cleaning tank 50 makes contact with a Freon vapor, which is
produced above a liquid level of Freon 51 by heating the Freon 51
up to 50.degree. to 60.degree. C. by means of a heater 52, and
thereby the mask 101 is subjected to vapor-cleaning. Still further,
the mask is quickly dried when it is taken out upwardly while
passing by a cooler 53 provided in the upper portion of this
cleaning tank 50. The cooler 53 prevents a Freon vapor produced in
the lower portion of the tank 50 from flowing out of the tank by
passing water at 10.degree. to 28.degree. C. through metallic
pipes, and thereby it suppresses consumption of Freon and also
quickly performs drying of a mask. This step of
vapor-cleaning-drying by means of an organic chemical gas can be
also achieved by means of isopropyl alcohol, and a similar effect
to that described above can be obtained by changing the set
temperatures of the heater 52 and the cooler 53. In addition, it is
to be noted that the vertical reciprocating motion of the mask in
the respective tanks as shown by arrows could be carried out either
once or a plurality of times, that is, it could be repeated any
times according to necessity. The mask cleaning section 2 in FIG.
5, which carries out the above-mentioned method of cleaning
according to the present invention, includes four cleaning tanks
20, 30, 40 and 50 for successively processing a mask to be cleaned,
and there are provided means for transporting masks to these
cleaning tanks as well as means for vertically reciprocating the
masks in the respective tanks. Furthermore, the respective tanks
are externally shielded from an external atmosphere, clean air
which has passed through a dust-removing filter 61 and an
electrolytically dissociated ion shower 62, is introduced into the
shielded space, while air is discharged through an exhaust manifold
63, and thereby maintenance of cleanness and prevention of
generation of static electricity within the cleaning section 2 can
be achieved.
According to the above-described embodiment of the present
invention, deposits and contaminations on the surfaces of a mask to
be cleaned are perfectly cleaned through the steps of
scrubbing.fwdarw.electrolyte cleaning.fwdarw.organic gas
cleaning.fwdarw.quick drying, substantially over the entire front
and rear surfaces. In addition, as compared to the heretofore known
cleaning process, the cleaning process according to the present
invention is short in time and excellent in workability, and it can
realize perfectness in the steps of the cleaning process as well as
reasonable cleaning treatment. Accordingly, the method of cleaning
according to the present invention is an especially advantageous
method for cleaning of a reticle mask, which necessitate dustless
and contaminationless cleaning.
Now an automatic dust inspecting section 3 that is available in the
illustrated embodiment of cleaning apparatus according to the
present invention will be explained in more detail with reference
to FIG. 6. A mask 6 to be inspected is held horizontally and moved
precisely in Y direction. Laser light from a laser head 17 is
reflected by the mirror 23 and irradiated to the front surface of
the mask 6. The most of incident laser light is changed as
reflected light 25 and is not detected by the photo-detector 18.
However, if residual dust 19 is present on the front surface of the
mask, the incident laser light is changed to scattered light 26 and
detected its largeness by the photo-detector 18. The mirror 23 can
scan the incident laser light in X direction at right angles with
the Y direction. Also, the condition of the rear surface of the
mask 6 is inspected by a laser head 17', a scanning mirror 23' and
a photo-detector 18'.
* * * * *